On-chip polarization splitters (PS) are essential in optical circuits such as advanced photonic integration circuits (PICs). Such polarization splitters are indispensable for many applications that need polarization diversity or other polarization manipulations. In terms of material systems, on-chip PS has been demonstrated in LiNbO3, polymer, glass, III–V semiconductors and other passive materials. Among these material systems, only III–V semiconductors are naturally suitable for active-passive monolithically integrated (APMI) applications. The various methods for making on-chip PS may be characterized as directional coupler or waveguide crossing based, asymmetric Y-branch based, Mach-Zehnder interferometer based, resonant tunneling based, multi-mode interference (MMI) based, and grating based.
In directional coupler or waveguide crossing based schemes, relatively large birefringence is used to make a directional coupler or waveguide crossing in bar state for one polarization and cross state for another. Asymmetric Y-branch based PS need asymmetric birefringence in two different waveguides and use mode evolution to ‘sort’ different polarizations into different waveguides. Mach-Zehnder interferometer based PS make inputs of different polarizations experience different optical path length difference in the interferometer so that they go to different output waveguides. Resonant tunneling based PS introduce a third waveguide in the middle of a directional coupler so that only one polarization is able to couple between two waveguides through tunneling of a middle one. MMI based PS terminate MMI coupler at imperfect imaging planes so that different polarization is able to couple to different output waveguides. Grating based PS take advantage of the fact that input of different polarizations will be diffracted to different spatial positions such that they may be separated.
However, such polarization splitters are not suitable for active-passive monolithic integration. They either rely on large material intrinsic birefringence which InGaAsP/InP material systems (i.e., for active function) do not possess, or they rely on air or metal cladding waveguides for larger birefringence, which are not compatible with low loss buried passive waveguides that can be integrated with active structures. Ultimately, it is preferred to have active functions, such as lasers, amplifiers, modulators, detectors and the like, monolithically integrated on a single chip with passive functions such as wavelength multiplexing/demultiplexing, polarization control, and signal filtering.